Issue 33

A. Shanyavskiy, Frattura ed Integrità Strutturale, 33 (2015) 8-16; DOI: 10.3221/IGF-ESIS.33.02

It was fractographically confirmed [7] that, at the microscopic scale level, the crack-growth behaviour is quite sensitive to the microstructure of the materials, and dislocation slip is dominated. Ivanova V.S. and Shanyavskiy A.A [4] have shown that the crack growth, at Stage I, in mesotunnels is associated with the development of slip: multiple-slip traces, slip steps or extrusion sites can be seen at the background of the pseudo-striations pattern. The fatigue crack propagation is quite fast in the mesotunnels at the Stage I. Consequently, the system experiences a self-organized transition to more complicated ways of energy absorption by the material, subjected to deformation, in which new free surface is being formed for meso-tunnels; this transition to the mesoscopic scale level (Stage II) occurs once the critical conditions at the crack tip were created [8]. The energy-absorption process becomes more complicated since the rotation effects are dominating in the deformation and fracture of the material at the meso-tunnel tip. Fractographic analyses of fatigue surfaces attest to Stage II (tensile Mode I) striation formation [4, 7], following Stage I crack growth. The dramatic decrease in crack growth acceleration between the two Stages is strongly exhibited by the kinetic (da/dN v  K) diagram for long cracks at the point of change in slope (or deviation) is witnessed under a regular cyclic loading condition. Therefore, a self-organized transformation from one form of energy absorption to another is occurring near to the crack tip. The shear mode of material separation (the mode II process) is the dominant mechanism of metal fracture below this deviation point whereas the opening mode (mode I process) is dominant above this point. This paper presents an analysis of the mechanisms involved in the formation of spherical particles at the mesoscopic scale level based on a rotation effect and the shear sliding process for aluminium based alloys. Both mechanisms were investigated fractographically, and, also, on the bases of the OG’e spectroscopy analyses. Let be consider a process of spherical particles formation in crosspieces between meso-tunnels. pherical particles wear formation under various cyclic loads conditions is well-known phenomenon [9-11]. They were discovered in compositions of wear debris are formed during rolling contact fatigue [9]. Further these particles were looking on the fretting surface [10, 11]. Fatigue cracks development in components or specimens is accompanied by processes of wear debris patterns formation on the fatigue surface because of crack edges interaction [4, 12-14]. The main idea for the interaction based on the process [13], which due to the mode II shearing of the mode I cracks growth under external tension loading for the near threshold fatigue cracks development. Rewelding occurs at the contact points across the crack as  I K falls [14]. Wear debris become detached from both fatigue surfaces during the rewelding and tearing processes. The leading role of the mode II K in contact points across cracks front have to be attracted to discuss the first stage of the fatigue crack growth because of the shearing mechanism which directed to the roughed surface formation. Suresh S. and Ritchie R.O [13] performed the roughness-model to calculate effective stress intensity factor for fatigue cracks growth under Modes  ( ) I II K K . The Mode II crack growth in specimens from Fe- and Al-based alloys was modelled under compressive cyclic loads, and spherical particles on the fatigue surface were shown [15]. There were places with wear debris of the black colour on the fatigue fracture surfaces for Al-based alloy. Two sizes for different particles shapes were discovered: (10...40)  m , and smallest than 10  m . The smallest particles were dominant. Small particles were often associated with small sockets and wear tracks, where the particles have been removed using replicating tape. The higher contrast suggested that they might contain a significant proportion of oxide. Their higher contrast in micrographs and their apparently greater hardness than the matrix tend to suggest that they contained a large amount of oxide. But it had not yet been possible to determine their exact composition. Various models [9-11] were discussed in the paper [15] and it was shown that they cannot explain a mechanism of the small particles formation. The well-known model [10] of wear particles formation by adhesive wear processes which trapped them in cavities in the sliding surface and became soothed by burnishing processes, can explain big particles formation only. According to the model, spherical particles to be anticipated in slow uniaxial sliding, in fretting and within cracks of a material being fatigued. Below results of the spherical particles fractographic analysis for fatigued specimens from Al-based alloys is discussed, and OG’e analysis uses to explain a mechanism of their creation during fatigue cracks growth. S S PHERICAL PARTICLES DUE TO M ODE III OF M ODE I FATIGUE CRACK GROWTH

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